1. Field of the Invention
The present invention generally relates to welding equipment and processes. More particularly, this invention relates to an arc welding process for repairing a compressor rear frame (CRF) of a gas turbine engine.
2. Description of the Related Art
FIG. 1 schematically represents a compressor rear frame (CRF) 10 of the CF6-50 high-bypass gas turbine engine manufactured by the General Electric Company. The frame 10 can be seen as comprising inner and outer casing walls 12 and 14 interconnected by radially-extending struts 16. A support flange 18 located at the forward end of the inner casing wall 12 is adapted to carry a stationary seal, referred to as the compressor discharge pressure (CDP) seal (not shown). When the frame 10 is installed in an engine, the CDP seal is positioned in close proximity to a seal disk (not shown) mounted on a shaft that interconnects the high pressure turbine and compressor of the engine. The CDP seal and seal teeth on the seal disk cooperate to create a tortuous flowpath between the inner casing wall 12 and the rotating shaft, thereby minimizing the amount of compressor discharge air bypassing the compressor downstream of the seal.
The support flange 18 inevitably requires replacement as a result of weld repairs performed on the frame 10 that cause distortion, resulting in the movement of the flange 18 to the extent that the dimensional limits of the flange 18 cannot be recovered. For this purpose, the inner casing wall 12 is typically machined to remove the flange 18, and a rough-machined annular-shaped flange is welded in its place. In this manner, the outer casing wall 14 and the bulk of the inner casing wall 12 can be salvaged. In addition to material considerations, which in this case is typically a nickel-base superalloy such as Inconel 718, suitable weld processes for welding the replacement flange 18 are dependent on the wall thickness of the inner casing wall 12 and flange 18 at the point where the weld is to be performed. Thin walls of up to approximately 0.090 inch (about 2.3 mm) can be welded by gas tungsten arc welding (GTAW) using a single-pass butt joint weld configuration. However, at the location where it is most practical to make the cut for removing a worn flange 18, the inner casing wall 12 of the CF6-50 engine has a nominal wall thickness of about 0.140 inch (about 3.56 mm). Under such circumstances, electron beam (EB) welding would typically be considered in combination with essentially the same single-pass butt joint configuration used with repairs preformed by GTAW. However, due to the set back of the CDP seal flange 18 inside the frame 10 and the proximity of the outer casing wall 14, EB welding cannot be used to perform this type of weld repair on the CF6-50 frame 10.
In view of the above, multiple-pass GTAW techniques have been developed, an example of which is represented in FIG. 2 as a detail of the seal flange 18 depicted in FIG. 1. As this approach is represented in FIG. 2, a weldment 20 is built up with four welding passes. Because of the limited penetration possible with conventional GTAW techniques, a butt joint is not used. Instead, a special joint preparation must be performed, such as the V-shaped channel 22 seen in FIG. 2. The requirement for multiple passes to build up the weldment 20 within the channel 22 is less than optimal in that performing such a repair is time consuming and often results in part distortions that can spawn a series of unplanned repairs. Weld-induced distortion exacerbates the existing tendency for the inner casing wall 12 to be out-of-round, which itself makes it difficult to perform the weld without necessitating machining of the wall 12 and flange 16 to the extent that the resulting wall thickness is below the allowable range.
A modified GTAW process, referred to as penetration-enhanced gas tungsten arc welding (PE-GTAW), has been recently developed that makes possible the welding of walls in excess of 0.090 inch. One such process makes use of a weld penetration-enhancing flux disclosed in U.S. Pat. No. 6,664,508 to Johnson et al. The flux, commercially available under the name Ni-139 from the Edison Welding Institute, Inc. (EWI), is disclosed by Johnson et al. as containing titanates and one or more transition metal oxides. It would be desirable if a single-pass GTAW processing utilizing a penetration-enhancing flux of the type disclosed by Johnson et al. could be adapted for use in welding compressor rear frames of gas turbine engines, and particularly the CRF of the CF6-50 engine.